Bicarbonate Reactive Silica (BRSi)
Procedure:
This procedure determines the total silica, as
bicarbonate-reactive silica, in filtered and unfiltered samples. The material which cannot pass through a 0.45
micron filter is considered particulate silica and measurable by the difference
between the filtered and unfiltered samples.
Silica reacts with a molybdate reagent in an
acidic environment to form a yellow silicomolybdate
complex. This complex is reduced by
ascorbic acid to form the molybdate blue color. The color intensity is proportional to the
silica concentration.
Interferences from phosphate, which forms a phosphomolybdate complex, and from tannin are eliminated by
the oxalic acid introduced in the sample stream before the addition of the
ascorbic acid reagent. Hydrogen sulfide
interference is removed by boiling during the autoclaving process.
Reagents:
For
sample preparation:
Sodium Bicarbonate-Digestion Reagent:
Sodium bicarbonate, NaHCO3 50
g
Milli-Q water, 1000
ml
Dissolve 50 g. of sodium bicarbonate in Milli-Q
and dilute to 1 L.
1 N Sulfuric Acid-Neutralization Reagent:
Sulfuric Acid,H2SO4 28
ml
Milli-Q water, 1000
ml
Add 28 mL H2SO4
to 900 mL MQ and dilute to 1 L.
For Autoanalyzer:
Ammonium Molybdate reagent:
Ammonium Molybdate reagent,(NH4)6Mo7O24.4H2O 10 g
Sulfuric Acid,conc. 2.8 ml
Milli-Q, water 1000 ml
Add 2.8 mL sulfuric acid to 800 mL of Milli-Q water. Dissolve 10 g ammonium molybdate
in the H2SO4 solution and dilute to 1 L. Filter, if needed, and store in an amber
plastic container at 4° C.
Ascorbic Acid:
Ascorbic acid 8.8 g
Acetone 25 ml
Milli-Q water 500
ml
LEVOR V 0.25 ml
Dissolve ascorbic acid in 250 ml. Milli-Q
water containing 25 mls acetone and dilute to 500 ml.
with Milli-Q. Store in an amber plastic container at 4° C.
Oxalic Acid:
Oxalic acid 25
g
Milli-Q water 500
ml
Dissolve oxalic acid in Milli-Q
water and dilute to 500 mls. Store in a plastic
container at 4° C.
Standards
The concentrations of silica in the Northern Temperate
Lakes LTER site vary greatly from lake to lake.
Thus, three ranges of standards are run for the lakes.
Low
range 2-70 ppb(Stds. 5-8
& blank)
Medium
range 70-1000 ppb (Stds.1-5, & blank)
High
range 1000-20,000 ppb (Stds.10-14,1 & blank)
Big Muskellunge, Crystal,
Crystal Bog and Trout Bog samples are analyzed as medium range. Sparkling, Trout, Allequash
samples and the bottom depth samples of Trout Bog are diluted 1:10 (1 ml
sample plus 9 ml. milli-Q water)
and are run as the high range along with the high range standards diluted 1:10. Trout Lake
station acid blanks are treated in an identical manner as the samples and are
analyzed with each concentration range.
Low BRSi samples (below 70 ppb) generally are
not reanalyzed at a higher sensitivity because of the high background produced
by the sample preparation procedure.
Silica Stock Solution: (145,000 µg/L)
Sodium silicofluoride, Na2SiF6 0.9719
g
Milli-Q water 1000
mL
Dissolve 0.9719 g. of sodium silicofluoride
in Milli-Q water and dilute to 1 liter. Store in a dark
polyethylene bottle at 4° C.
Stability: Indefinite.
Working Standards
Diluent: 1%(v/v)
Ultrex HCl in Milli-Q. Use a
polyethylene volumetric flask to prepare the diluent! All volumes are recorded as weights by
preparing the standards on the AE200 Mettler
balance. See labbook
for further details.
High Range: (made
from Silica Stock)
Std. 10: 10 mls. Silica
Stock, dilute to 100 mls with diluent. Std 11: 6 mls. Silica stock, dilute to 100 mls. with diluent.
Std. 12:
4 mls. Silica Stock,
dilute to 100 mls. with
diluent.
Std. 13
2 mls.
Silica Stock, dilute to 100 mls.
with diluent.
Medium Range: (made
from Std. 10)
Std 1: 6 mls. Std. 10,
dilute to 100 mls. with diluent.
Std. 2:
4 mls. Std 10,
dilute to 100 mls. with diluent.
Std. 3:
2 mls. Std.10, dilute
to 100 mls. with diluent.
Std. 4:
1 mls. Std. 10, dilute to 100 mls. with diluent.
Low Range:(made
from Std 2)
Std. 5: 12 mls. Std
2, dilute to 100mls. with diluent.
Std. 6:
7 mls. Std. 2, dilute to 100 mls. with diluent.
Std. 7:
4 mls. Std. 2, dilute to 100 mls.with diluent.
Std. 8: 1 ml. Std.
2, dilute to 100 mls. with diluent.
A standard blank is prepared from the 1% Ultrex HCl used to
dilute the standards.
Sample Preparation:
--Both unfiltered and filtered samples are run.
1. Aliquot
10 ml sample to polysulfone “Oak
Ridge” style centrifuge tubes. (Shake samples before aliquoting.)
2. Add 3.5 mL 50g/L
NaHCO3 to each tube.
Cap with polypropylene cap.
Vortex
3. Loosen
caps to vent. Tighten caps. Repeat after 30 minutes.
4. Autoclave
90 minutes at 115-120° C.
5. Cool to
room temperature.
6. Add 2 mL 1 N H2SO4. Vortex. Wait 30 minutes, and vortex again. (Want to insure that all CO2 is
blown off to avoid complications while running on the autoanaylzer.) After each vortex, loosen caps to vent. Tighten caps.
7. Run 10%
replicates through procedure.
8. Run 10%
blanks through procedure
Run
Set-up
1. Fill up
tray with flat-bottomed cups.
2. Medium
range is run first:
Positions
1-5 are Standards 1-5
Position
6 is standard blank
Positions
8-10 are TLBL’s (if necessary)
3. High
level is run second:
Positions
1-6 are Standards 10-14 & 1
Position
7 is Blank
4. Low level is run third:
Positions
1-4 are Standards 5-8
Position
5 is Blank
Instrument
conditions:
The
Silica module is installed after the pump and the 660 nm. filters
are inserted into the colorimeter. The
standard calibration setting on the colorimeter is typically around 1.0 for BRSi analysis.
The Houston Omniscribe
recorder settings are as follows:
chart speed: 5 cm/min
toggle switch: ÷10
range: .01 V
Dissolved
Reactive Silica (DRSi) Standard:
This procedure determines the reactive silica in filtered samples Silica
reacts with a molybdate reagent in an acidic
environment to form a yellow silicomolybdate
complex. This complex is reduced by
ascorbic acid to form the molybdate blue color. The color intensity is proportional to the
silica concentration.
Interferences from phosphate, which forms a phosphomolybdate complex, and from tannin are eliminated by
the oxalic acid introduced in the sample stream before the addition of the
ascorbic acid reagent.
Reagents:
Ammonium Molybdate reagent:
Ammonium Molybdate reagent,(NH4)6Mo7O24.4H2O 5 g
Sulfuric Acid,conc. 1.4 ml
Milli-Q, water 500 ml
Add
1.4 mL of sulfuric acid to 400 mL
of MQ. Allow solution to mix then dissolve 5 g of ammonium molybdate
in this solution and dilute to 1 L.
Filter, if needed, and store in an Erlenmeyer flask in refrigerator.
Ascorbic Acid:
Ascorbic acid 8.8 g
Acetone 25 ml
Milli-Q water
500 ml
Aerosol 22 0.25
ml
Dissolve ascorbic acid in 250 ml. Milli-Q
water containing 25 mls acetone and dilute to 500 ml.
with Milli-Q. Then add 0.25 mL
of Aerosol 22. Store in an amber plastic container at 4° C.
Oxalic Acid:
Oxalic acid 25 g
Milli-Q water 500
ml
Dissolve oxalic acid in Milli-Q
water and dilute to 500 mls. Store in a plastic
container at 4° C.
Standards:
The concentrations of silica in the Northern Temperate
Lakes LTER site vary greatly from lake to lake.
Thus, three ranges of standards are run for the lakes.
Low
range 2-70 ppb(Stds. 5-8
& blank)
Medium
range 70-1000 ppb (Stds.1-5, & blank)
High
range 1000-20,000 ppb (Stds.11-14,1 & blank)
Big Muskellunge, Crystal,
Crystal Bog and Trout Bog are run as medium range. Then any sample that has a peak height lower
than Std 5 (70 ppb) must be reanalyzed on the low
range. Sparkling, Trout, Allequash samples and the bottom depth samples of Trout Bog
are diluted 1:10 (1 ml
sample plus 9 ml. milli-Q water)
and are run as the high range along with the high range standards diluted 1:10. The
acid blanks from Trout Lake station are treated in the same
manner as the samples and analyzed on
medium and low level ranges.
Silica Stock Solution:
Sodium silicofluoride, Na2SiF6 0.9719
g.
Milli-Q water 1000
mls.
Dissolve 0.9719 g. of sodium silicofluoride
in Milli-Q water and dilute to 1 liter. Store in a dark
polyethylene bottle at 4° C.
Stability: Indefinite.
Working Standards:
Diluent: 1%(v/v)
Ultrex HCl in Milli-Q. Use a
polyethylene volumetric flask to prepare the diluent! All volumes are recorded as weights by
preparing the standards on the AE200 Mettler
balance. See the labbook
for details of the standard preparation procedure.
High Range: (made
from Silica Stock)
Std. 10: 10 mls. Silica
Stock, dilute to 100 mls with diluent. Std 11: 6 mls. Silica stock, dilute to 100 mls. with diluent.
Std. 12:
4 mls. Silica Stock,
dilute to 100 mls. with
diluent.
Std. 13
2 mls.
Silica Stock, dilute to 100 mls.
with diluent.
Medium Range: (made
from Std. 10)
Std 1: 6 mls. Std. 10,
dilute to 100 mls. with diluent.
Std. 2:
4 mls. Std 10,
dilute to 100 mls. with diluent.
Std. 3:
2 mls. Std.10, dilute
to 100 mls. with diluent.
Std. 4:
1 mls. Std. 10, dilute to 100 mls. with diluent.
Low Range:(made
from Std 2)
Std. 5: 12 mls. Std
2, dilute to 100mls. with diluent.
Std. 6:
7 mls. Std. 2, dilute to 100 mls. with diluent.
Std. 7:
4 mls. Std. 2, dilute to 100 mls.with diluent.
Std. 8: 1 ml. Std.
2, dilute to 100 mls. with diluent.
A standard blank is prepared from the 1% Ultrex HCl used to
dilute the standards.
Run
Set-up
1. Fill up
tray with flat-bottomed cups.
2. Medium
range is run first:
Positions
1-5 are Standards 1-5
Position
6 is standard blank
Positions
8-10 are TLBL’s (if necessary)
3. High
level is run second:
Positions
1-6 are Standards 10-14 & 1
Position
7 is Blank
4. Low level is run third:
Positions
1-4 are Standards 5-8
Position 5
is Blank
Instrument
conditions:
The
Silica module is installed after the pump and the 660 nm. filters
are inserted into the colorimeter. The
standard calibration setting on the colorimeter is typically around 9.7 for DRSi analysis.
The Houston Omniscribe settings are as follows:
chart speed: 5
cm/min
toggle switch:
÷10
range: .01 V for
medium and high samples
.001 V for low range samples
Sample Preparation:
There is no elaborate procedure for DRSi sample preparation such as that of the BRSi analysis. The filtered samples are poured directly
into the AutoAnalyzer II cups and analyzed.
Last revised: 8/24/97 by James Thoyre